1. Field of the Invention
This invention relates to the stylus used as a positional indicator in a coordinate input device, which is an input device of a computer system, normally called a digitizer or tablet, and more particularly to a stylus that can detect pen touch pressure or which comprises pen pressure sensitive mechanisms.
2. Description of the Prior Art
A stylus pen used as a positional indicator of the digitizer is normally provided not only to detect the coordinates of an indicated position, but also to detect pen-down information where the pen point is in touch condition with the position detecting surface. Generally, the method to detect the position includes detecting a depressing force loaded against a member provided at the top of the pen, which is the component transmitted along the axial direction of the pen.
First, the method for detecting the position of the digitizer will be explained before describing the arrangement of the stylus. There are several positional detecting methods used in the digitizer, so an example using an electromagnetic method for receiving/transmitting the information will now be described. It has combined arrangements of a positional detecting plane on which many sensor coils are positioned parallel to the direction of the positional detection, and a positional indicator such as a pen or cursor. Two arrangements are normally provided in the X and Y directions by orthogonally positioning them relative to each other on the respective detecting plane to detect both in the X and Y directions. A coil or resonant circuit is also provided to make electromagnetic interactive operations between the sensor coils. To point out an example, electromagnetic waves are sent out from the sensor coil, and responsive electromagnetic waves, which are radiated because of an interactive action between the resonant circuit within the positional indicator and the electromagnetic waves, are received at the sensor side. Operations, including interpolations, are performed, based upon the signal received from the coil that has the most powerful signal strength and its neighboring coil, to decide the position thereof.
A feature of this electromagnetic receiving/transmitting method is that it can be fabricated, and the positional indicator is a cordless device and there is no need for a power source within the indicator. Detailed description of it can be found in Japanese Patent Application laid open H2-53805 and H3-147012.
There is also another method for detecting position in which electromagnetic waves are sent from a positional detecting plane and are received at a corresponding positional detector, and a further simplified electromagnetic method in which electromagnetic waves are sent from a positional indicator reverse to those of a former wave, and are received at the positional detecting plane side.
The positional indicator, as mentioned above, using the electromagnetic interaction to detect the respective position, is provided with coil or resonant circuits within itself. Furthermore in that positional indicator of the stylus type, it has been realized these operations change the resonant conditions of the above-mentioned coils or resonant circuits from that of the stable conditions.
Several arrangements have been proposed for a stylus which has a stylus pressure sensitive mechanism. For example, an arrangement was provided in which part of the inductance or capacitance of the resonant circuit or coil was replaced with a variable coil or capacitance, or to which a variable resistance was added so that these variable components were allowed to produce a certain change when the stylus pressure was applied thereto. Reference should be made to the Japanese patent application Publication Laid-open Nos. S63-257823, S64-532222, H1-5322, H4-96212, and H5-275283.
Some prior art examples of the pressure sensitive styli are shown in FIG. 1A, FIG. 1B, and FIG. 1C, in each of which a resonant circuit is disclosed.
As for commonly provided structural components in the styli shown in FIGS. 1A to 1C, they comprise a casing 12 having a cylindrical outer body, a core body 14 provided on the axis of the casing, a ferrite core 16 having a through-hole for slidable housing body 14, coil 20 wound around the core 16, a movable magnetic body that can move in relation to the core, an elastic body or spring 22, and a condenser(capacitor) 24. The core body 14 has a generally cylindrical form, while its upper neighboring portion which touches the positional detecting plane has a tapered form such that the operation allows it to easily indicate a specified point. Other components such as optionally provided switches are not shown.
The principle of pressure detecting operation during the pen-down mode will now be described. The ferrite core 16 is fixed to pen casing 12. The body 14 moves backward along is axis by is depressing force when the pen top 32 is depressed against the positional detecting plane. The moveable magnetic body 18 is positioned to move in conjugation with the core body in the examples. The relative distance to the core 16 is therefore varied as the magnetic body 18 moves. In FIGS. 1B and 1C, the symbol di indicates the initial gap (in case any depressing force exists) between the core 16 and the magnetic body 18, while the coil 20 and the capacitor 24 form a resonant circuit. Conditions of this resonant circuit are determined such that the circuit resonates with the sensor coil while receiving and transmitting the electromagnetic waveforms between them. The inductance of the coil 20 can be changed when the gap between the core 16 and the magnetic body 18 is changed. It thus detects the depressing force against the pen top by sensing the change in the resonant conditions caused by where the pen is placed.
In FIG. 1A, the magnetic body 18 is fixed onto the side wall of the core 14 so that it moves with the core 14. In FIG. 1B, the body 18 is fixed to the rear end portion of the core 14, so that it moves in accordance with the axial movement of the core 14. In FIG. 1C, the magnetic body 18 is placed in front of the core 16, so that it is fixed in a manner to surround the core 14.
A feature common to each of the styli 10 of the prior art shown in FIGS. 1A to 1C is that the core body 14 has a structure in which it moves only in its axial direction. That is, in any of the examples, the core 14 moves only in a sliding manner through the hole of the core 16 of the fixed magnetic body. Sufficient gain is attained with this arrangement if the stylus 10 is operated in vertically standing condition relative to the positional detecting plane, since the depressing force effects toward its central axis.
However, if the stylus 10 is operated in an inclined position against the detecting plane, the direction of the depressing force is then no more coincident with the axial direction of the body 14. In such a case, only the axial component of the depressing force is transferred to the body 14 so that the amount of the depressing force contributing to detection is decreased. Thus, the inductance change generated by the displacement of the magnetic body is reduced. The sensitivity of the pen pressure detection is also reduced if the stylus is operated in an inclined position so that it cannot detect a weaker pen pressure. If it is necessary to achieve the same level of sensitivity as that of the pen when orthogonally disposed relative to the plane, although the pen is placed in inclined position, an increased depressing force would have to be exerted for the pen to get the same displacement of the core body, i.e., magnetic body as that of the aforementioned orthogonal position to the plane.
Normally, it is natural for an operator to hold the pen in an inclined position, rather than in an orthogonal position while inputting with the pen in hand onto a horizontal plane.
To attain an increased absolute sensitivity by compensating for such a defect, it is desirable to provide structure that provides the moveable magnetic body 18 with a larger area, and has its end face opposed to the face of the fixed magnetic body 16 such as shown in FIG. 1B rather than to use the body 18 having a smaller area shown in FIG. 1A. In FIGS. 1B and 1C, it is however required to assemble the pen with the initial gap predetermined as the interval between the two magnetic bodies when the pen is not depressed by its tip. That requires high accuracy when placing the moveable body.
Ferrite material normally used for the fixed magnetic body is made through sintering, so that its dimensional tolerance is low and thus it is difficult to ensure the accuracy mentioned above.
A second problem in operating the stylus with an inclining position is the friction encountered when the core body slides through the hole cut into the ferrite core, so that the pen pressure sensitivity is decreased. Also in this regard, it is necessary to depress the pen with a much increased force if the pen is used in an inclined position.
Thus, in all regards, the operability of the stylus in an inclined position is low. Therefore, an input operation with a light operational feeling is not realized. This makes the operability lower, not only with the positional detecting device, but also with the entire computer system.